Interior concrete slab guide: vapor, moisture, and flatness
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An interior concrete slab has a job a driveway never gets. It has to take a finish on top, like a coating, a polish, tile, or vinyl glue. Three numbers decide how that goes. You want a 10 to 15 mil vapor barrier under the concrete, a moisture reading under 3 pounds per 1,000 square feet, and FF 50 flatness. Miss one, and the fix costs more than doing it right would have. Thickness, base, and mix basics are in the concrete flatwork guide.
Why does an interior slab take more care than a driveway?
Nobody polishes a driveway. Outdoor flatwork gets a broom finish and no vapor barrier. Its flatness is loose enough that nobody measures it, because the surface is the product and the weather is the only critic. A polished floor indoors gets held to FF 50 and a moisture reading under 3 pounds.
Indoors, the slab hands off to whatever goes on top of it. Moisture that would dry right off a patio has nowhere to go once a floor covering seals the top. And a ripple nobody sees on a garage apron looks like a wave when side light hits a showroom floor.
So the finishing changes too. The outdoor slab gets a rough broom finish, because texture gives grip and nobody is going to grind it. The interior slab gets a steel trowel, a sealed barrier under it, and a tight flatness target. Then it gets a water cure, which leaves no film on top to block the bond. Interior concrete flatwork uses the same concrete and the same crew as a driveway. The choices change from the dirt up.
Does an interior concrete slab need a vapor barrier?
Yes, and this one isn’t up for debate. Any slab inside a heated or cooled space gets one, whether your plans call it an interior slab or an internal slab. It’s a 10 to 15 mil polyethylene sheet, a heavy plastic, laid right under the concrete on top of the rock base before the pour. The seams lap six inches and get taped. Every pipe and conduit that comes up through it gets sealed.
The sheet costs a few hundred dollars on a typical home slab, and skipping it costs far more. Epoxy and polyaspartic coatings peel off (delaminate). Tile and vinyl glue lets go. White stains called efflorescence bloom on the surface, and a musty smell shows up that nobody can trace. Fixing it means grinding off the failed finish, drying the slab, and coating it again, at many times what the sheet would have cost. Ground moisture doesn’t stop moving because a slab got poured over it. It rises, and whatever you put on top becomes the lid it pushes against.
A sand layer over the barrier is a mistake
An old detail puts a few inches of sand between the vapor barrier and the concrete. It was meant to give bleed water from the mix somewhere to go. It does the opposite. That sand fills with water from the mix, from rain during the build, from anything at all. With the barrier under it, the water can’t drain down. So it sits there and feeds moisture into the slab from below for years.
The barrier goes right under the slab, not under sand. If a plan sheet still calls for this “blotter layer,” talk it through before the pour, not after the floor gets coated.
How do you test an interior slab for moisture before finishing?
Two standard tests settle it, and they measure different things. ASTM F1869 is the calcium chloride test. A measured dish of water-free calcium chloride sits sealed on the slab for 60 to 72 hours. The weight it gains turns into a moisture vapor emission rate (MVER), in pounds per 1,000 square feet per 24 hours. ASTM F2170 puts humidity probes in holes drilled to 40 percent of the slab depth. They read what the concrete is doing inside. That matters on a thick slab, because the top can read dry while the core is still wet.
| Test | What it measures | Pass mark | When it is used |
|---|---|---|---|
| ASTM F1869 (calcium chloride) | Moisture leaving the surface (MVER) | Common pass: 3 to 5 lb/1,000 sf/24 hr | Quick check of the top |
| ASTM F2170 (in-situ RH) | Humidity inside the slab | Below the coating maker’s RH limit, often 75 to 85% | The surest check before a coating |
| Both together | Top plus core moisture | Both must pass | Big-ticket floors, or a warranty on the coating |
Many coatings list a pass mark of 3 to 5 pounds per 1,000 square feet per 24 hours. We put a moisture barrier primer down first when a test reads over 3. New concrete usually needs 28 days or more to read under 3, and that wait can’t be squeezed. It matters when a general contractor asks if the floor can be coated next week, or when the pour date slides and the finish date doesn’t.
A slab feels dry to your hand long before a probe says it’s dry. That gap is where most coating failures start.
We check every slab. On a new pour, or when the check shows a warning sign, the test comes before the coating goes down. Run it after the floor starts to let go, and the number only tells you why you’re paying twice.
What do FF and FL actually measure?
FF is floor flatness and FL is floor levelness, and ACI 117 defines both. A crew reads them with a profiler, such as a Dipstick or an inclinometer cart. FF tells you how bumpy the floor is over short distances. FL tells you how much the whole room tilts. They’re separate numbers. A slab can be dead level and still ripple, or dead flat and still run downhill.
A standard troweled pour lands around FF 25 to 30. A slab headed for polished concrete wants FF 50 or higher. That gap is the difference between grinding to refine a floor and grinding to reshape it.
Flatness is cheap on pour day and costly after. A laser screed, a good finish crew, and honest time on the trowel all buy you FF points. Each point you get that way is one you don’t pay for later with diamonds and a week of labor. And you never fully buy it back. Grinding a wavy slab flat cuts deep into the cream, the thin cement layer on top, and sometimes into the stone. That changes the look of the floor, whether you wanted it or not.
What sits under an interior slab?
The standard stack runs from the bottom up. First comes the packed-down soil (the subgrade), then 4 to 6 inches of crushed rock base, then the vapor barrier, then the concrete. The rock base stops ground water from wicking up into the slab, and it gives the slab a firm base that drains. Packing the soil is the step people rush, because it doesn’t show up in a photo.
Two cases add a layer. A heated slab with radiant PEX tubing cast into the concrete needs rigid foam under it, or you’re paying to warm the dirt. A slab in a cold climate needs the same foam for the same reason. Use extruded or expanded polystyrene rated to at least 25 psi of compressive strength, which is the load it takes before it crushes. Plan the foam and any radiant tubing before the forms go up. Neither one can go in after the pour. And foam that crushes under the pour is worse than no foam. Now you’ve got a soft spot and a low spot in the same place.
Prep for polish vs. prep for a coating
Both finishes start from the same slab. But each one wants the slab handed over in a different shape.
Polished concrete wants a hard-troweled slab at FF 50 or better. It should be poured with little water in the mix, which is a low water-to-cement ratio. That means a dense surface, tight cream, and no film on top. It’s why a water cure matters here. It leaves a clean surface the diamonds can bite. An acrylic curing compound has to be ground off completely before the polish can even start.
Epoxy flooring and other coatings want grip instead. The crew diamond-grinds or shot-blasts the slab rough. The coating maker names how rough, as a concrete surface profile (CSP) range. On a new slab, or one that shows a warning sign, the moisture reading is the gate. The slab has to read under the system’s limit, by MVER or by humidity, or the coating doesn’t go down that day.
The four mistakes that cost the most
A sand blotter layer over the vapor barrier. It traps water with no way out and feeds the slab from below for the life of the building.
Skipping the moisture check because the slab looks dry. Looking dry is only the surface. Coating over a wet core is how a floor peels six months after everyone signed off.
Acrylic curing compound used because it’s easy. It cures the slab and blocks the bond. Every square foot has to come back off before a finish will stick.
Taking a standard FF 25 pour for a floor that will be polished. That choice locks in extra grinding and a wavy reflection. It gets made months before anybody notices, usually by someone who was never told the floor would be polished. The concrete flatwork guide covers the pour-side basics that keep this from happening.
Pouring a slab that will be polished or coated?
Tell us the square footage, the finish you want, and where the site is. We’ll spec the vapor barrier, the flatness target, and the cure, so the slab is ready to finish the first time. You get a straight read on what your floor needs. The quote tool prices the drive from the team nearest your site.
Full scope, pricing, and the spec tiers for the pour itself are on the interior slab service page.
